Detecting and locating fluid leaks beneath slab-on-grade foundations is often a difficult and destructive task. Because the slab blocks access to the soil underneath, one must either break up the slab to inspect the subjacent soil or drill relatively large holes through the slab to bore soil samples. Further, these methods require a certain amount of guesswork because of the uncertain location of suspected leaks. Consequently, a number of holes must be drilled or even jack hammered through the slab before the actual leak is located.
A less destructive method of detecting and locating leaks beneath slab foundations involves directing a radar signal down through the slab and into the subjacent soil. Because the radar return from wet soil will differ from that of dry soil, using radar may allow one to approximate a leak location without partially destroying the slab. However, certain types of reinforced slab foundations may attenuate the radar signal sufficiently such that an accurate return from the subjacent soil is unobtainable. Moreover, radar is not well suited to detecting small leaks, nor for differentiating between small leaks and variations in soil composition or the presence of roots and other plant matter.
Methods currently exist for detecting and locating leaks from landfills, hazardous waste dumps, impoundments, and other outdoor fluid containment areas by measuring changes in the conductivity and/or resistivity of the adjacent soil. Daily et al. '406 discloses mise-a-la-masse and electrical resistance tomography leak location methods. Mise-a-la-masse involves driving an electrode within a fluid containment facility to an electrical potential with respect to another electrode placed at a distance from the facility. Voltage differences are then measured between various combinations of additional electrodes placed in the soil adjacent to the facility. The leak location is located by determining the coordinates of a current source pole that best fits the measured potentials within the constraints of the known or assumed resistivity distribution. Because the potentially leaking fluid must be driven to a potential, mise-a-la-masse methods can monitor for leaks in continuous fluid systems only, such as ponds, lined fluid containment areas, and tanks.
Electrical resistivity tomography (ERT) involves placing electrodes around the periphery of, beneath, or, in the case of subsurface containment vessels, above the facility. A known current is applied to alternating pairs of electrodes, then the electrical potential is measured across other alternating pairs of electrodes. From this data the electrical resistivity at a plurality of points in the soil can be calculated. Disturbances in resistivity will correlate with migration of leaking fluid. However, Daily does not disclose a method or apparatus that allows the electrodes to be placed directly under the leak source, after construction of a building or other structure.
Henderson '202 and '045 both disclose directly monitoring the soil subjacent to a fluid containment area by burying electrodes directly beneath the containment. Both Henderson patents disclose a plurality of four-plate electrode systems. A voltage and a known current are applied across the outer pair of plates. The resulting potential difference is measured across the inner pair. Henderson '045 also discloses a system of individual electrodes that, by varying the spacing between the electrodes impressing a current into the ground and the spacing of the potential measurement electrodes, can indirectly measure the resistivity at a calculated depth. However, Henderson '045 does not disclose a method of directly monitoring the subgrade beneath a structure without permanently burying the electrodes or a method to place electrodes beneath an existing structure.
Woods et. al '244 discloses a leak detection system for radioactive waste storage tanks. The system comprises a metal tank, an AC generator connected between the tank and a reference electrode, and a plurality of reference electrodes. When the generator is energized, it creates an electric field in the ground between the tank and the reference electrode. A voltmeter measures the potential difference between the sensing electrodes and the tank. A significant change in the potential at one or more of the sensing electrodes indicates that the tank has developed a leak. Woods et al. has a number of disadvantages. First, it requires an electrically conductive fluid container. Second, it requires that the electrodes be permanently buried in the soil surrounding the tank. Finally, it requires the use of an AC generator, which is less convenient than a DC power source.
None of the prior art is entirely satisfactory to locate fluid leaks beneath an existing slab foundation. For instance, it is not practical to electrify the potentially leaking fluid--typically a plumbing system--and because there may exist multiple sources of fluid, mise-a-la-masse is not a practical option. Nor is it practical to embed permanently a series of electrodes beneath the slab to monitor soil resistivity. Further, because some of the ERT methods--for example, the Henderson references--use multiple-plate electrodes, a large hole would have to be bored into the slab to insert the electrodes into the subjacent soil making the method impractical and destructive. In addition, placing the electrodes around the periphery of the foundation is less accurate compared to placing the electrodes directly beneath the potential leak source. All the prior art electrical mapping methods only map a two-dimensional area, assuming resistivity to be uniform with depth, rather than a three-dimensional volume. Also, all the prior art methods are static. That is, once in place, none of the electrodes are moved to generate different or three-dimensional data.
Another potential problem with using existing electrical mapping methods such as ERT to locate leaks beneath a slab foundation is "non-uniqueness." Non-uniqueness is an ambiguity that arises in data. For instance, a large resistivity anomaly at a large distance from an electrode may generate the same data as a smaller anomaly located close to the electrode. This ambiguity, or non-uniqueness, may mischaracterize the actual subsurface resistivity profile, resulting in a resistivity map that indicates leaks or other resistivity anomalies in incorrect locations.
Accordingly, a need exists for a method for locating fluid leaks and other anomalies beneath slab-on-grade foundations without the need for destroying the slab or permanently burying electrodes beneath the slab. Another need exists for access to the soil beneath the slab as unobtrusively as possible. Further, a need exists for an electrical resistivity mapping method and apparatus for detecting leaks beneath a slab foundation without requiring the electrification of the fluid source. Additionally, a need exists for a method and apparatus to map a three-dimensional subjacent volume's electrical resistivity with both static and dynamic electrodes. Finally, there is a need for a method and apparatus to minimize or eliminate the errors associated with "non-uniqueness."